A conventional spark plug is mounted on an internal combustion engine by screwing a thread ridge formed on an outer circumference of a metal shell into a female screw formed on a mounting hole of an engine head of the internal combustion engine. Such a spark plug includes an annular sealing member (a gasket) provided on the outer circumference of the metal shell in order to prevent an air leakage from a combustion chamber through the mounting hole. A conventional gasket is formed from an annular shaped cold-rolling strip (hereafter referred to as “Fe”). The annular strip is folded back in the radial direction so as to assume, for example, an “S” shape in the cross section. When screwing the spark plug into the mounting hole, the gasket is sandwiched and compressed between a projecting portion of the metal shell and an opening circumference edge portion of the mounting hole and is deformed to thereby provide a seal therebetween. An axial force (reactive force in the axial direction due to compression caused by tightening the spark plug) acts on the gasket. As a result, the air leakage from the combustion chamber through the mounting hole is sealed.
As internal combustion engines have been miniaturized and advanced in recent years, engine vibration tends to increase, and a temperature in the combustion chamber tends to rise. Because a gasket made of the conventional Fe has relatively low durability over a creep deformation, which is caused by heating and cooling cycles during an engine drive and stop, the spark plug mounted on an engine tends to come loose, resulting in deterioration in the axial force. Therefore, a gasket made of stainless steel, which has a higher rigidity than that of Fe and is unlikely to cause the creep modification, is employed to secure the air tightness of the combustion chamber. (For example, see Japanese Patent Application Laid-Open (kokai) No. 2004-134120).
However, along with the miniaturization of internal combustion engines, spark plugs have also been miniaturized. Since a metal shell of such a sparkplug is formed slimmer and its durability becomes low, a recommended tightening torque when mounting the spark plug is also set to be low. Since a gasket made of stainless steel with a high rigidity is unlikely to plastically deform, sufficient axial force after tightening the spark plug cannot be obtained when the tightening torque is low. As a result, the air tightness in the combustion chamber becomes insufficient. On the other hand, when the tightening torque is raised in order for a gasket to sufficiently deform plastically, stress exerted to a thread neck of a metal shell, which has a low durability due to its miniaturization, increases, resulting in a possible fracture or the like of the spark plug.
The present invention is accomplished in order to solve the above-mentioned problems, and an object of the present invention is to provide a spark plug and a sealing member for the spark plug capable of providing a sufficient axial force with a low tightening torque.